I just bought a 1992 Corvette and the speedometer is reporting erratic.
This happens driving and also sometimes when the car is running sitting still. All of the other gauges work and I accessed the mode in the air conditioning control to mode 7 and it show the same speed as the cluster. The radio was missing and a harness was just laying in the stereo bay so I thought I would plug the wires back into the stereo unit under the dash on the drivers side.
I have replaced the VSS and I still have the problem.
I have also ran a new ground wire from the VSS directly to the battery and that did not help either. I also cleaned the contacts at the VSS and pulled the connectors off the ECM and cleaned them even though they looked perfect. Last week end I took the car for a drive and I noticed that I am getting a squeaking noise coming from the Right rear wheel that I think is the Hub assembly so I will be changing that (could the speed sensor in the hub cause this ?) Like I was saying I tool it for a drive last weekend and took the car into a car wash because it was nasty from just sitting and I took it thru twice and on the second time thru the speedometer started working and worked for the rest of the day ( not sure if that was from the water or not) but later that night after it sitting for a few hours it stopped working.
I am pulling my hair out on this can someone help.
The cruse is not working and the car sometimes will not start without having to give it a little gas. Don't know if that info helps but I thought I would mention it. Also it has a Genuine GM ECM Refurb in it.

 

There's a module behind the radio that sends out the speed signal. If the wires are tight on the connection is corroded or dirty or mother wise less than good, this can happen.

Cruise control is driven off a vacuum system. Typically a vacuum leak is the most common cause of cruise not working.

 

Thank you...do you know the name and actual location of this box ?
Are you referring to the CCM?

 

CCM sounds right.

 

I think I found it. I will pull it out and clean it and give it a try.
I will let you know thank you

 

When you say the speed readout is erratic what do you mean. Is it reading random numbers or are the digital just flashing and jumping around like a pin ball machine> When the speed reading is erratic what is the display doing when you press 7.

 

Hello,
Everything in the display is working the only problem is the speed readout it may say 220 when sitting still or 1 while doing 60. The odometer barley advances since most of the time it is reporting 5 mph or less. This also affects the range calculations and average and instant mph numbers.
The display on the air conditioner in mode 7 shows the same speed as the cluster.
Hope that helps.

 

Sounds like a CCM problem or wiring. Maybe corrosion on the CCM connector. If the dash and the control panel both read the same the signal comes from the VSS to the CCM and is displayed on the dash. Did you check the connector on the VSS I have seen the connector get oil soaked and the rubber break down and short the pins together.

 

OP - The VSS needs to be wired as shown in the FSM. You mention some things about grounds to battery etc that make absolutely no sense. If it's not wired as in the FSM the rest of what you're checking seems worthless.

Get the wiring back to OE, remove the VSS and spin with a rotary tool to see what you can confirm OR since you mention the newer makes no difference try it with the old.

Do you have issues other than the VSS? Maybe but until you return all wiring to OE you can't diagnose. Do you have the FSM (Factory Service Manual)?

 

Thank you for your thoughts. I did remove the extra ground wire and the problem is still there.
I did dig into the dash and pull the CCM and cleaned the connectors and reseated the connectors a couple of times. I Also pulled the cluster and cleaned that connector also.
All of the contacts looked like new. I also cleaned the connectors on the ECM a couple of nights ago.
I did notice that both the ECM and the CCM are both have AcDelco reman stickers on them.
I don't have a manual so I may have to break down and buy one.
The car seams to have a small starting issue when you start it sometimes you have to give it just a little throttle and it lights right off and stays running.
Other than that everything else seams to be working fine.
I see that they just replaced the optispark and the battery and the water pump and radiator in the last few months, but I have not idea how long the Speedo has had the problem.

 

Hello,
Saturday I pulled the dash apart to get the CCM (the CCM had an AC Delco Reman sticker on it) out of the car to clean the contacts. They looked really clean but I cleaned them anyway as well as the contacts going into the digital dash. I put the car back together and I was still getting the erratic numbers at the speedometer.
Sunday my wife and I took it for a drive for about 8 hours on and off with about a 120 mile round trip. After driving the car about 10 or 15 miles that speedometer started working and was displaying the exact speed that my Navigation speed was showing. From time to time either by hitting a bump in the road or shutting the car off and going in a store and coming back out it would stop working and then after a little bit it would come back on start working. When I say come back on the display would be displaying correctly other times is was just jumping from 2,5 55,220 etc.
I think this may be a bad connection or broken wire so. I am going to pick up a new VSS connector tonight and change the end out. If that still does not work I will run 2 new wires I think to the ECM. I am purchasing an FSM but if someone knows the colors and pins etc where the other end of that VSS harness goes it would be great to hear from you.
Also any other ideas?

 

One day my speedometer was going crazy, the mileage didn't even move. I check the ECM and the chip inside became unseated. I snapped it back in and everything was fine again.
Hope you can solve your problem as easy as I did.

 

Thank you for that thought. I just picked up a new connector for the VSS and I plan on cutting the old one off and soldering and shrink wrapping on the new one tommorrow after work. The current one is 25 years old and is really dirty and oily. So I thought it was worth a try. I will also pill the ECM and check the chip. If that does not work i will probably replace the vss wire up to the ECM.
Wish me luck! I will post my findings good or bad.

 

What brand and part# did you use for the replacement VSS? Do you still have the old original VSS from the car?

 

Hello,
Yes I do still have the original vss and I still have the box the new one came out of. The part number for the vss for that year I was told by the dealer was discontinued and he gave me another number that was discontinued also. I ended up getting the new one from NAPA I can get you the number if needed.
But the old one and the new one are going me the same results. I would think that if the Speedo works for 30 mins or so and it is accurate when working makes me think the new vas is good. The old thing I can think of was that I had to flip the metal bracket that holds the vss into the transmission it 180 degrees out and that would have made the plug in a different spot so I flipped the bracket!
The black one is the original and the white one is the new one!
This is the inside of the vss opening!

 

I think my thread got mixed up with another thread.
Anyway I went ahead cut off the original connector off the vss under the car and soldered on another one.
Test drove the car and it is still erratic.
I have the FSM now and I see that the ckt 400 and 401 are the 2 wires going into to the Brown ECM connector from the vss, and there is a ckt 817 is the output from the ECM going to the ccm.
I am thinking I should replace the 2 wires coming from the vss to the ecm. If I cut the wires going into the ECM and solder my 2 new wires in will that hurt anything?
I don't have a vss speed generator to test the signal at the end plug. Any ideas on how to do this?
Thanks again for all your help...
Oh also I did pull the ecm and pull the prom and reinsert it. Both the ECM and the CCM are ac Delco rebuilds.

 

OK first of all I made a couple hundred dollars mistakes by misdiagnosing.
This what I did to fix it.

1. Replaced the VSS (No Change)
2. Replace the VSS connector (No Change)
3. Traced and tested VSS to ECM wires for SHORT or OPEN(No Change)
3. Found and Purchased an Original CCM ( Not a rebuild ) (No Change)
4. Figured it had to be the ECM considering I changed everything else.

I found a couple ECM rebuilders and they had never heard of a rebuild fixing the speedometer issue.
I had read to try to get another ECM other than the original, because most likely another one would not have the same issue.
I called around and everyone wanted $375.00 -$400.00 for an original not rebuild ECM.
So I decided to take the ECM board out of the housing and resolder the connections because that is what most of the rebuilders do and get rid of the old COLD SOLDER JOINTS.
After I did I put the ECM back in and Speedometer started working fine as well as the Cruise control.
If you don't want to take the time to do that just get one from a wrecking yard.

Hope this helps....

 

This is the exact problem that I had on my 92, speedo would jump all over the place. Sometimes it would work and most of the time it would be all over the place.
While trying to diagnose the problem, I had it in park and the speedo was reading 0. I had a friend tap on the ECM and it would start reading crazy. Replaced the ECM and it worked fine.